A fuel cell is configured as a stack by stacking a plurality of unit cells together with separators (also called bipolar plates) interposed therebetween which have grooves serving as channels for gases (hydrogen, oxygen, etc.), the unit cells each including an anode electrode, a cathode electrode, and a polymer electrolyte membrane sandwiched therebetween. The output of such a fuel cell can be heightened by increasing the number of cells per stack.
Separators for fuel cells serve also to permit the generated electric current to flow into adjoining cells. The separator materials constituting separators are hence required to have both high electrical conductivity and conductive durability which enables the high conductivity to be maintained over a long period even in the high-temperature acidic atmospheres within the cells of the fuel cells. The high conductivity and conductive durability mean that the contact resistance is low. Contact resistance means that a voltage drop occurs due to an interfacial phenomenon between the electrode and the separator surface.
Separator materials developed so as to satisfy those requirements are proposed, for example, in Patent Documents 1 to 4.
Patent Document 1 describes a separator for polymer electrolyte fuel cells which includes a metallic base material having an oxidized coating film of the base material itself formed in the surface thereof and a conductive thin film formed over the surface of the oxidized coating film, in which an interlayer for enhancing adhesion has been formed between the oxidized coating film of the base material itself and the conductive thin film, the conductive thin film being a thin carbon film constituted of carbon (C) and formed on an atomic level, and the interlayer is constituted of at least one layer of a layer (Me) constituted of one or more elements selected from among metals consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W and among semimetallic elements consisting of Si and B, and a (carbon-Me) gradient layer which has been formed on the (Me) layer and includes carbon (C) and metallic or semimetallic element(s) (Me) and in which the proportion of carbon (C) increases as the distance from the base material increases.
Patent Document 2 describes a separator for fuel cells which includes a base material made of a metal and a surface treatment layer formed on the surface of the base material, in which the surface treatment layer includes a base material-side portion constituted of metallic or semimetallic element (Me) or a carbide of the metallic or semimetallic element (MeC) and an opposite-side portion from the base material which is either a portion constituted of carbon (C) and formed on an atomic level or a portion constituted of (C+Me or MeC) obtained by compositing carbon with metallic or semimetallic element (Me) or a carbide of the metallic or semimetallic element (MeC) on an atomic level.
Patent Document 3 describes a separator for fuel cells characterized in that it includes a metal plate whose surface has been coated with a conductive carbon film, the conductive carbon film is one formed by a chemical vapor-phase synthesis method or a sputtering method at a film deposition temperature of 400° C. to 600° C., and the conductive carbon film has a density of unpaired electrons of 1020 electrons/cm3 or higher, a G/D ratio as determined by Raman spectrometry of 0.5 or less and a resistivity of 10 Ωcm or less.
Patent Document 4 describes a separator for fuel cells which includes a metallic base material and a coating layer formed thereon, characterized in that the coating layer includes an amorphous carbon layer and a conductive portion.